Recent comments from SciRate

sattath Sep 29 2016 11:15 UTC

Here is the second (more informative) abstract:
We introduce a new quantum cryptographic primitive which we call
a tokenized signature scheme. Such a scheme can be used as an ordinary
digital signature scheme, with the additional property that the signer
can produce and distribute one-use quantum si

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Barbara Terhal Sep 28 2016 19:35 UTC

Many thanks for the clarification! I think I see your point (we had a similar issue with needing to have high overlap with input and output penalty in the universal adiabatic construction in my paper with Gosset and Vershynina, using the space-time construction).

Elizabeth Crosson Sep 28 2016 16:57 UTC

Thank you for the question! This construction due to Peres is interesting, but if I'm analyzing it correctly then I don't think it would work in the context of our paper. The ground state probability distribution of the Hamiltonian with couplings in Peres eq (20) looks like the ground state of a d

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Barbara Terhal Sep 28 2016 14:09 UTC

In http://journals.aps.org/pra/abstract/10.1103/PhysRevA.32.3266 Asher Peres showed how to modify the Feynman Hamiltonian to make sure that a Hamiltonian evolution starting at t=0 lands after some fixed time at the desired output time so that the Hamiltonian effectively corresponds to that of large

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HA Sep 26 2016 15:35 UTC

Thanks!

Tom Wong Sep 26 2016 14:15 UTC

The supplemental material is in the arXiv source. Once you extract the tarball, it's under anc/supplemental_material.pdf.

HA Sep 22 2016 18:51 UTC

The supplemental material is missing! It would great to see the LP optimisation method used.

Marco Piani Sep 19 2016 20:13 UTC

Is it actually decidable? :-)

Toby Cubitt Sep 19 2016 15:00 UTC

I like this sentence from the conclusion: "There is, however, a second possible answer to our question: yes".

Māris Ozols Sep 15 2016 21:30 UTC

Here is a link for those who also haven't heard of SciPost before: https://scipost.org/

Zoltán Zimborás Sep 15 2016 18:12 UTC

This is the very first paper of SciPost, waiting for the first paper of "Quantum" (http://quantum-journal.org). There are radical (and good!) changes going on in scientific publishing.

James Wootton Sep 14 2016 07:46 UTC

"Ni." would be slightly shorter, but some may find it offensive.

Frédéric Grosshans Sep 13 2016 12:38 UTC

The abstract my be a bit too long.

climaiw Sep 12 2016 16:19 UTC

I would suggest planets where life can really make a difference.

Omar Shehab Sep 12 2016 12:50 UTC

I am still trying to understand the following statement from II.A.

> This leads to the condition that the first- and second-order moments
> of the model and data distributions should be equal for the parameters
> to be optimal.

Mohammad Bavarian Sep 08 2016 03:58 UTC

So beautifully written!

Māris Ozols Sep 07 2016 13:03 UTC

John also has an excellent series of 7 blog posts covering this material:
https://www.physicsforums.com/insights/struggles-continuum-part-1/

Samad Khabbazi Oskouei Sep 05 2016 11:34 UTC

I think that we have missed the "semi-" at the conclusion. Because, the proof of the theorem 4.3 is based on the using universal semi-density matrix concept which is not computable. The semi-computability concept used here is like the Kolmogorov complexity which is not computable and so the Cubic co

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Toby Cubitt Sep 01 2016 11:14 UTC

I could well be missing something. But as far as I could tell from a rather quick read through the paper, all they show is that the quantum capacity of a channel with computable matrix elements is given by the regularised coherent information optimised over input ensembles with computable matrix ele

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Māris Ozols Aug 30 2016 17:52 UTC

Do I understand correctly that this paper claims to show that quantum capacity is computable?

> After defining the algorithmic quantum capacity we have proved that it
> equals the standard one. Furthermore we have shown that it is
> computable.

resodiat Aug 23 2016 13:00 UTC

That is really a long-term perspective.

Marco Piani Aug 22 2016 22:08 UTC

Born in Italy, and now living in Scotland: I have no excuses not to feel inspired :-)

Māris Ozols Aug 22 2016 18:50 UTC

It is not just in Scotland but in fact across the whole of UK and even beyond. I just found a reference, dating back to the very birth of quantum computing, where the early pioneers [already admit][1] that their work was inspired by Rabezzana Grignolino d'Asti.

[1]: https://scirate.com/arxiv/quan

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James Wootton Aug 18 2016 16:42 UTC

A video of a talk I gave this morning will be [here][1], if it ever finishes uploading.

[1]: https://youtu.be/I8cMY0AmIY0

Jonathan Oppenehim Jul 28 2016 16:41 UTC

Hi, sorry to just be updating this discussion now -- my conversation with Renato seemed to me to have converged here (and also continued via email and in person and I never updated scirate). However, a few people have asked what the outcome of our discussion was. So let me just say, that yes, my vie

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Valentin Zauner-Stauber Jul 18 2016 09:54 UTC

Conjugate Gradient IS a Krylov-space method...

Renato Renner Jul 08 2016 06:07 UTC

I completely agree with your analysis, which describes the gedankenexperiment from a global (“outside”) perspective, according to the laws of Bohmian Mechanics (BM). And, indeed, it shows that the "memory" of a measurement outcome cannot assumed to be permanent, i.e., it may change (according to BM)

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Tony Sudbery Jul 02 2016 19:09 UTC

Roger Colbeck drew our attention to this paper in the York QFIT group, and we met to discuss it last week. I would like to comment on the relation of Bohmian quantum mechanics to the extended Wigner's friend experiment. As generalised by John Bell, Bohmian qm can be applied to this experiment to yie

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Lidia del Rio Jun 26 2016 17:51 UTC

Video of a *journal club* session about this paper, with a 20min explanation of the paradox and a long audience discussion about implications and interpretations:

http://pirsa.org/16060101/

Notation: "for simplicity" some names have been revealed (like the friends'), some have been changed

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Mario Krenn Jun 08 2016 06:58 UTC

Too bad, the paper has been withdrawn due to a mistake :-/

Renato Renner Jun 07 2016 16:09 UTC

Maybe. But if your solution was not only self-consistent but also "single-world" then it would certainly contradict our theorem.

@Jonathan: I am unsure whether the concern you had is related to ($\star$), too, but I would anyway be interested to know whether it is this type of statements that you

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André Xuereb Jun 06 2016 08:07 UTC

If I had a self-consistent answer to that I'd be able to solve the measurement problem, no?

Renato Renner Jun 06 2016 07:47 UTC

OK, then we have identified the point where we disagree. Our assumption (QT) indeed corresponds to ($\star$), together with the understanding that the state is physically relevant.

Having said this, I am still curious to know how, in your opinion, experimenter F1 should derive her prediction ab

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André Xuereb Jun 03 2016 09:45 UTC

Yes, I would say so.

Renato Renner Jun 03 2016 09:42 UTC

Could one then summarise your viewpoint as follows: Statement ($\star$) by itself is correct, but the state $\psi$ of the system is not the one that is physically relevant (i.e., we should not use it to deduce claims about the outcomes of future measurement on the system, for instance)?

André Xuereb Jun 03 2016 06:18 UTC

I think our point of disagreement is not $(\star)$ in itself, but rather the relevance of $\psi$ that enters this statement. I would also make the second bullet point stronger.

- When discussing erasure and subsequent measurements, one needs to keep track of both $E$ and $S$, even if they are no

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Renato Renner Jun 03 2016 05:34 UTC

Yes, it may be easier to chat about this in person. But since I know a few who are following this discussion, let me at least summarise the points where I agree with you:

- In a scenario where experimenters are themselves subject to quantum measurements, it is not allowed to describe them using c

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André Xuereb Jun 02 2016 20:47 UTC

The point I was originally trying to make was that the experimenter must be treated within the same framework as the system. The argument I was trying to make was that it is not always justified to treat the experimenter as classical (what I said above regarding the environment can be rephrased to t

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Renato Renner Jun 02 2016 20:08 UTC

The example itself makes sense to me. However, I am unsure how it connects to the questions we were discussing.

Certainly, it illustrates that the statement ($\star$) mentioned earlier would be wrong if there was a non-trivial exchange Hamiltonian governing the interaction between experimenter a

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André Xuereb Jun 02 2016 08:20 UTC

In a sense, yes, that's what I'm saying.

Let me take a situation that is very familiar to me: Cavity QED. We model a single-mode field inside a cavity with leaky mirrors by means of an interaction Hamiltonian like $\hat{H}_\text{int}=\mathrm{i}\hbar\int\mathrm{d}\omega\ \kappa(\omega)\bigl[\hat{b}(

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Renato Renner Jun 02 2016 07:36 UTC

Just to make sure I got this correctly, are you saying that (at least in certain situations) the predictions we obtain when we use quantum theory depend on where we make the split between quantum system and measurement device? (This would raise the question whether there is a rule that tells us wher

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André Xuereb Jun 02 2016 06:53 UTC

I won't attempt to solve the general problem. At least not without some beers.

My point was directed at the particular argument at hand.

- I have a laboratory $L$ inside which is a quantum system $S$ and an experimenter $E$
- $E$ performs some measurements on $S$, but $E$ is not treated as part

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Renato Renner Jun 02 2016 06:40 UTC

I agree with what you wrote in the last paragraph — you may even regard this as an implication of our Theorem 1 (at least provided that you take SW for granted).

I nevertheless have difficulties making full sense of the position you take. If I understand it rightly, the ultimate conclusion would

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André Xuereb Jun 02 2016 06:05 UTC

The ability to impose a "cut" and isolate a system (e.g., a single H atom) from its surroundings and treating that system as a quantum object disconnected from its surroundings, or connected to them solely by means of classical channels, is an extremely convenient fiction. But it is, at the end of t

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Renato Renner Jun 02 2016 05:29 UTC

My previous statement must have been ambiguous. But when I wrote that an experimenter applies quantum theory to a system, I did not mean to imply that she applies classical theory to its surroundings. Rather, the idea was that she can use QT to study any arbitrary subsystem of the universe, provided

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André Xuereb Jun 01 2016 19:52 UTC

My understanding of quantum theory is precisely that: It must be applied to the experimenter themselves as well as the apparatus. I come from the school of thought that believes that everything is quantum.

As per my previous comment, within this framework it may be possible that your paper could

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Renato Renner Jun 01 2016 19:15 UTC

Indeed, our argument is based on the idea that any experimenter is free to use quantum theory to describe the world *around* her (but not including herself). If I now understand correctly, you are saying that this is not in general a correct use of quantum theory. Did I get this correctly?

Alexander Belov Jun 01 2016 16:26 UTC

I don't think that QIC is to blame here. ECCC is also not indexed by google scholar for some strange reason. Try this paper http://eccc.hpi-web.de/report/2016/073/ for instance.

André Xuereb Jun 01 2016 13:51 UTC

You are perfectly right; that is exactly what confused me.

Personally, then, I feel slightly uneasy with saying that your property QT captures quantum mechanics, since I believe strongly that quantum mechanics must describe the state of the experimenter themselves. This is especially pertinent in

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Renato Renner Jun 01 2016 13:33 UTC

I agree with what you are writing about $\psi_L$. While the state of the prepared system $S$ does not depend on whether $E$ remembers it, the state $\psi_L$ (which includes $E$) does.

I am not sure though what you meant when you wrote that $\psi_L$ is the "relevant object". Note that property QT

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